Endoscopic systems or microscopic systems (which will be generically referred to as imaging systems) are known as systems that image biological tissue of patients to allow the biological tissue to be observed at the time of surgical operations or inspections. Some of the imaging systems have autofocus (AF) functions of automatically bringing objects into focus. For example, Patent Literature 1 discloses a microscopic system that detects a light amount change in the light from an object, drive an optical system to execute an AF operation in a case where the light amount change falls within a predetermined range, and moves the optical system to an initial position without performing any AF operation in a case where the light amount change is beyond the predetermined range.